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repair broken sewer or gas utility lines, near leaking, underground storage tanks, or any place
that the soil could reasonably be expected to contain this type of hazard. Testing should be made
prior to entry into the trench. Further testing should continue as conditions warrant.
Acceptable atmospheres for trenching operations are:

• Oxygen Level: 19.5% - 23.5%
• Flammability: Less than 10% of LEL (Lower Explosive Limit)
• Toxicity: Less than the PEL (Permissible Exposure Limit for that chemical)

Ventilation may be required to keep the atmosphere parameters within acceptable limits. Extra
caution and continuous monitoring should be provided in the situation where continuous
ventilation is required to keep the hazardous atmosphere from building back.

Many people wonder if the area within a trench could also be a confined space. OSHA has stated
that the only time a trench area would be considered a permit-required confined space is when
there is an actual or potential hazardous atmosphere present. In that case, the trench may also
be classified as a permit-required confined space.

Emergency Equipment

Hazardous atmospheres and dangerous trench designs require that the workers entering the
trench wear equipment which will safeguard them from the atmosphere within, will affect their
rapid removal, and/or will assist in their emergency medical treatment. This type of equipment
must be well-maintained and must be attended when in use.

Water Hazards

Water is one of the greatest hazards in trench work. It can undermine the trench walls or decrease
the soil's ability to bear weight. Further, transmitted water pressure can turn the bottom of the
trench into quicksand. In any case, the probability of wall failure is greatly enhanced by
accumulating or seeping water. No worker is allowed in a trench where water is seeping or
accumulating unless adequate precautions against the water hazard are in place. Therefore, if
work must take place in the trench, the Competent Person has the responsibility to design and
monitor the water removal process.

Water Removal

In many cases, the water that accumulates in a trench is water that is natural drainage and
seepage from the groundwater table. Factors determining how deep the groundwater table will
be include soil type(s), elevation of the trenching site, recent rainfall, and natural or man-made
mechanisms of drainage (tile). The groundwater table should ideally be kept at least 2 feet from
the floor of the trench. Dealing with water from below the soil surface can be accomplished in the
following ways:

Local Pumping

In many cases, the water inside the trench can easily be pumped from the trench. The end of the
suction line is placed in the lowest portion of the accumulated water and water is drawn through
the pump and out a discharge line. The advantage of this system is its simplicity. One
disadvantage of local pumping is that while it will remove gross standing water, it will not leave

Trench & Excavation
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